Automotive The reason for the low efficiency of the ICE - zero shoulder crankshaft

[vochka]

I don't know English well, translated using google translate.

The working mixture ignites at TDC, and at this point the crankshaft looks up (0 degrees). As the piston descends, the force that the piston needs to travel a certain distance (for example, 1 mm) decreases, up to the moment when the knee rises at 90 degrees. Further, on the contrary, - the force that the piston needs to travel a certain distance increases, up to the moment when the knee rises at 180 degrees.

The pressure of gases at TDC (0 g) is maximum, decreases due to an increase in volume as the piston is lowered, and does not remain constant, since the time of flame propagation at TDC can be neglected.

It turns out that the internal combustion engine uses the energy of combustion very ineffectively; they also include new "developments" with a rail instead of a connecting rod. Why ineffective? Therefore, as most of the energy (in this case, heat), created in the range of 0-90 degrees, the engine spends on heating, or rather on the forced removal of heat.

My idea: At the TDC point, the crankshaft knee should be approximately 80 degrees. Because this is the only way to achieve maximum efficiency.

 

[jrmichler]

Your engine has zero compression and zero efficiency because the piston smashed the cylinder head completely off the engine when the crankshaft passed 80 degrees before top dead center. Either that, or the connecting rod was destroyed.

Ideas about linkages should be checked by drawing the system to scale, with a separate drawing at regular intervals.

 

[russ_watters]

It turns out that the internal combustion engine uses the energy of combustion very ineffectively; they also include new "developments" with a rail instead of a connecting rod. Why ineffective? Therefore, as most of the energy (in this case, heat), created in the range of 0-90 degrees, the engine spends on heating, or rather on the forced removal of heat.

My idea: At the TDC point, the crankshaft knee should be approximately 80 degrees. Because this is the only way to achieve maximum efficiency.

The Otto cycle thermodynamic efficiency has basically nothing to do with the mechanics of the crankshaft. When you do the basic analysis to predict efficiency you don't even use the mechanics of the engine, just the operating states/points in the cycle. What you are doing here just isn't how thermodynamics works.

 

[Nugatory]

The working mixture ignites at TDC, and at this point the crankshaft looks up (0 degrees)... the time of flame propagation at TDC can be neglected.

The propagation time cannot be neglected, it takes a fair number of milliseconds from ignition to peak pressure as the flame front spreads through the cylinder. On the engine that I am most familiar with (Fiat SOHC) at 6000 RPM we have only ten milliseconds per revolution so we arrange to fire the spark plug at somewhere around 30 degrees before TDC, and peak pressure is reached about ten degrees after TDC. At lower speeds the crankshaft is turning less quickly relative to the flame front so less spark advance is needed; I generally go for about ten degrees at 1000 RPM idle.

 

[jrmichler]

The engine in my truck has computer controlled ignition timing. The exact ignition timing, and other variables, can be read through the diagnostic port. So naturally, I have a gadget on the dash (ScanGauge) that does exactly that. As @Nugatory said above, the ignition timing depends on RPM and manifold pressure. The timing in my engine varies from 46 degrees before TDC to 2 degrees after TDC.

 

[russ_watters]

The engine in my truck has computer controlled ignition timing. The exact ignition timing, and other variables, can be read through the diagnostic port. So naturally, I have a gadget on the dash (ScanGauge) that does exactly that.

I have one too, but in a drawer now. If you want to be geekier (and cordless), consider a Bluetooth adapter and smartphone app like Torque.

 

[vochka]

The propagation time cannot be neglected, it takes a fair number of milliseconds from ignition to peak pressure as the flame front spreads through the cylinder. On the engine that I am most familiar with (Fiat SOHC) at 6000 RPM we have only ten milliseconds per revolution so we arrange to fire the spark plug at somewhere around 30 degrees before TDC, and peak pressure is reached about ten degrees after TDC. At lower speeds the crankshaft is turning less quickly relative to the flame front so less spark advance is needed; I generally go for about ten degrees at 1000 RPM idle.

Thanks for the answer, Nugatory.
Since this entire discussion has already been conducted on the Russian forum, I will give some calculations from there.

Given:
3000 rpm - 50 rpm / sec
Piston stroke 90 mm
Flame front speed 30 m / s

Downstroke time = 1/100 sec
The propagation time of the flame front in the cylinder behind the piston is equal to a distance of 90 mm, i.e. 0.09m divided by the speed, i.e. 30m / s. Equal to 1/300 sec

At 3000 rpm, the flame spreads 3 times faster.
Naturally, there is nothing to say about idling. But at 6000 rpm, the flame propagation speed is already equal to the piston stroke speed.

 

[Nugatory]

Given:
3000 rpm - 50 rpm / sec
Piston stroke 90 mm
Flame front speed 30 m / s

Downstroke time = 1/100 sec
The propagation time of the flame front in the cylinder behind the piston is equal to a distance of 90 mm, i.e. 0.09m divided by the speed, i.e. 30m / s. Equal to 1/300 sec

At 3000 rpm, the flame spreads 3 times faster.
Naturally, there is nothing to say about idling. But at 6000 rpm, the flame propagation speed is already equal to the piston stroke speed.

That's not how any of this stuff works. There is no single piston stroke speed; instead the piston speed is near zero during the dwell time at the top and bottom of the stroke and reaches a maximum at about 90 degrees ATDC (I say "about" because the calculations are a bit tricky and depend among other things on the wrist pin offset). The speed of flame front propagation is determined by several variables: mixture atomization and distribution, combustion chamber geometry, the piston/head squish area, mixture richness, and mixture pressure..
To get decent energy efficiency (as opposed to volumetric efficiency, which also matters for any pracical engine) we want the highest practical compression ratio and ignition timing such that peak cylinder pressure is reached when the piston is past TDC and well into its downstroke. (And note that in any modern engine doing four-digit RPMs the exhaust valve is going to open well before the bottom of the stroke).

A good reference, now somewhat dated, is Charles Taylor' s two-volume "The internal combustion engine in theory and practice"; Philip Hubert Smith's "The design and tuning of competition engines" is also good and much more readable.

 

[vochka]

we want ... such that peak cylinder pressure is reached when the piston is past TDC and well into its downstroke.

How many percent of the total piston stroke is enough for you? The question is rhetorical. I don't need all this. I want the peak cylinder pressure to be at the moment when the crankshaft shoulder is at 80 degrees, approximately. Then, as in bicycle pedals, the heat loss to the engine walls will be minimal, although there is no heat loss in bicycle pedals, since the pressure there is not created by the heated gas, which is surrounded by a 2000-degree cold wall.
Thank you. Sorry for my English, I think I can make a mess of something.

 

[Nugatory]

I want the peak cylinder pressure to be at the moment when the crankshaft shoulder is at 80 degrees, approximately.

You don't get that result by changing the geometry of the piston/crank/conrod, you get it by designing the combustion chamber, ignition timing and cam timing to produce the peak pressure at the right time in the crankshaft rotation.

 

[vochka]

You don't get that result by changing the geometry of the piston/crank/conrod, you get it by designing the combustion chamber, ignition timing and cam timing to produce the peak pressure at the right time in the crankshaft rotation.

If the combustion rate would be ten times less, then I would have been able to get the maximum pressure at the time of rotation of the crankshaft in the 75-80 degrees, as you write, "by designing the combustion chamber, ignition timing and cam timing to produce the peak pressure at the right time in the crankshaft rotation."
But with real speed, your proposal is impossible. It is necessary to change the crank mechanism so that when the piston is in the upper position, the crankshaft is already rotated by 75-80 degrees, so that the gas pressure energy can be used as much as possible. In this case, heat losses will be minimized.
Thanks anyway.

 

[pbuk]

My idea: At the TDC point, the crankshaft knee should be approximately 80 degrees.

This is impossible. The connecting rod is rigid and so when the piston is at the top of its travel ("Top Dead Centre") what you call the "crankshaft knee" is also at the top of its travel.

Ideas about linkages should be checked by drawing the system to scale, with a separate drawing at regular intervals.

Yes. In this case just one more drawing will do: draw the crankshaft 80° earlier (i.e. rotated anti-clockwise) so that the crank is at the top of the crankshaft. Make sure the connecting rod, piston and cylinder are the same size as the original drawing. Where is the piston?

 

[vochka]

I really cannot accept that the forum participants do not understand what this is about. They really think that I'm not going to change anything in Crank mechanism. I repeat, I am not asking how to do this, the question is different.

I argue that if engineers succeed in creating an engine in which the piston, being at TDC, will turn the crankshaft turned by 75-80 degrees further up to 150-160 degrees, then the efficiency of this engine, due to a decrease in heat losses, which occurs due to the fact that a good moment of pressure increase in the combustion chamber occurs in an unsuccessful turn of the crankshaft, will increase by 30%.

Question - prove that this will not happen. The question of creating a mechanism is not yours or my business, if it is necessary, then creating such a mechanism is as easy as shelling pears.

For example, in 2017 on the Russian forum, as soon as I created a similar topic, the fourth message was answered: "Either I did not understand something, or ..." and he inserted pictures, the scraps of which I inserted in my first message hoping that everything is clear from the text of the first post. Now I am showing the same picture with an English translation. Of course this is not a solution. The user just wanted to understand what I mean.

 

[jrmichler]

Enough is enough. The OP has been told several times, by different people, how to analyze his idea. He has been told that it cannot work, and why. He refuses to listen. Thread closed.